Chronic itch is a significant clinical problem associated with many dermatological conditions and systemic kidney and liver disease. Most forms of chronic itch are resistant to antihistamine treatment. A better understanding of itch mechanisms is urgently needed to identify cellular targets for development of novel anti-itch treatments. We propose a rodent model that distinguishes between itch and pain. Itch is usually assessed by hindlimb scratching directed toward a site of application of itch mediators in the nape of the neck of rats or mice. However, because of biomechanical limitations, the only response available to the animal is directed movement of the hindlimb to the stimulus site. Therefore, a drawback of the method is that other sensory qualities besides itch are likely to elicit scratching. In a new model involving stimulation of the cheek, at least two motor responses are available: hindlimb scratches and forelimb wipes directed to the stimulus site. Intradermal cheek injection of histamine, which is itchy to humans, elicits almost exclusively hindlimb scratching in rats and mice. In contrast, capsaicin, which is painful, elicits almost exclusively ipsilateral forelimb wiping. We propose to rigorously test if these distinct responses distinguish between itch and pain, and to investigate the neural processing of these sensory qualities. Thus, specific aim 1 will test if a variety of itch-producing chemicals selectively elicit facial scratching with the hindlimb, and if various pain-producing chemicals selectively elicit facial wiping with the forelimb. This aim will also test if scratching is reduced by drugs that block 5-opioid receptors but not by morphine, and if wiping is reduced by morphine but not by drugs blocking 5-opioid receptors. Specific aim 2 will investigate underlying neural mechanisms by recording responses of second-order trigeminal neurons to itch- and pain-evoking stimuli delivered the facial skin receptive field. We will test the hypothesis that itch- signaling neurons respond to itchy chemicals over a time course matching that of facial scratching behavior. Finally, specific aim 3 will exploit this model to investigate sensitization of itch-signaling pathways under conditions of chronic itch produced by dry facial skin. We will test the hypothesis that itchy chemicals will elicit greater scratching, while pain-evoking chemicals will elicit scratching instead of wiping due to a modality switch from pain to itch in sensitized itch-signaling pathways. An improved understanding of facial itch and pain mechanisms will provide targets for the development of means to interrupt itch and pain transmission, with great potential for translation to clinical treatment of these common conditions. PUBLIC HEALTH RELEVANCE: Chronic itch is a significant clinical problem associated with many dermatological conditions and systemic diseases, and is usually resistant to antihistamine treatment. This project will investigate a rodent model that distinguishes between facial itch and pain, and will address the underlying neural mechanisms. The project will improve our understanding of itch mechanisms and identify cellular targets for translational development of new treatments for itch and pain.